FIELD OF THE INVENTION
The present invention relates to a method of producing arabitol by continuous fermentation of at least one sugar by micro-organisms producing arabitol.
It relates more particularly to a method of producing arabitol by continuous fermentation of at least one sugar by micro-organisms producing arabitol, employing at least two interconnected fermentation areas.
To be more precise, in the method according to the invention, arabitol is produced in a first fermentation area in such a manner that some of the sugar introduced into the fermentation medium that it contains is consumed, some of said fermentation medium is transferred into a second fermentation area, the volume of said first area being maintained constant by adding sugar, and production of arabitol continues in the second a fermentation area in such a manner as to consume the residual sugar.
Thereafter, in the second fermentation area, the fermentation medium with a low residual sugar content obtained in this way is separated into a fraction concentrated in micro-organisms and another, soluble fraction rich in arabitol, possibly with recycling of said micro-organisms to the entry of the first fermentation area.
The industrial preparation of arabitol is based mainly on fermentation processes.
Generally speaking, micro-biological methods of producing arabitol can employ two different fermentation modes: a batch mode and a continuous mode.
In the batch mode, two techniques are conventionally used: the batch fermentation technique and the fed batch fermentation technique.
In the batch fermentation technique, all the substrates necessary for feeding the micro-organisms are introduced at the beginning of the fermentation and the arabitol produced is extracted at the end of fermentation.
The term "substrates" refers to all the nutrients that are introduced into the fermentation medium. In the context of the invention, the substrates are mainly carbonaceous sources (including sugar) and nitrogenous sources which can be assimilated directly by the arabitol-producing micro-organisms.
However, this continuous fermentation technique has the drawbacks of requiring long fermentation times to enable total consumption of the sugar introduced into the fermentation medium and of necessitating the use of large volumes of fermentation medium, and therefore large fermenters, to obtain satisfactory conversion yields. This leads to a mediocre productivity per unit volume.
Moreover, variation of the concentration of one of the ingredients of the fermentation medium can affect the arabitol yield or productivity.
One solution is to use the second batch fermentation technique, known as fed batch fermentation.
In particular, this technique introduces the substrates progressively into the fermentation medium.
The advantage of this technique is that the substrate concentration in the fermentation medium can be controlled and all the sugar can be consumed.
Thus the fed batch fermentation technique solves problems associated with the potential inhibiting effects of some ingredients of the fermentation substrates on the growth of the micro-organisms or on the production of arabitol or, for example provides finer adjustment of the aeration that conditions arabitol production.
However, to obtain satisfactory yield and productivity, it is necessary to use high substrate concentrations. Most importantly, the fermentation times are long and the various fermentation steps require complex and painstaking control.
The method described by ESCALANTE et al. (in Journal of Fermentation and Bioengineering, 70-4, 228-231, 1990) for the production of arabitol by Hansenula using fed batch fermentation cannot achieve industrially viable yield and productivity.
This is because, although arabitol is selectively produced, and although the fed batch fermentation process ensures that all of the glucose introduced into the fermentation medium is consumed, arabitol is obtained with a yield of the order of only 14%.
The second fermentation mode conventionally used for the biological production of arabitol is the continuous mode, as described by JAKOBUS van ZYL and PRIOR in Appl. Microbiol. Biotechnol. (1990), 33, 12-17, for example.
In continuous mode, all the fermentation substrates are added continuously to the fermenter and fractions of the fermentation medium are extracted at the same flowrate as the introduction of substrates, to maintain a constant working volume.
This fermentation mode significantly increases yield and productivity but does not produce a product of high purity because the arabitol drawn off is necessarily contaminated by the substrates re-introduced during fermentation.
Consequently, the major drawback of continuous mode fermentation processes is the need to purify the arabitol. Also, available purification techniques can separate the arabitol from the residual sugar not assimilated by the micro-organisms only with difficulty.
For this reason, complex, large and costly purification installations are conventionally associated with these continuous fermentation processes.
Fermentation experts therefore devote all their efforts to seeking operating conditions which can reconcile improved arabitol yield and productivity with a low residual sugar content enabling easy and low-cost purification of the arabitol produced.
Thus the aim of the present invention is to solve the problem of producing arabitol of satisfactory purity with excellent yield and productivity by employing a particular continuous fermentation technique.
The method of producing arabitol developed by the Applicant company consists of fermenting at least one sugar using arabitol-producing micro-organisms in two fermentation areas.
The invention relates more particularly to a method of producing arabitol by continuous fermentation of at least one sugar by micro-organisms producing arabitol, characterised in that:
a) arabitol is produced in a first fermentation area including at least one fermenter in such a manner that some of the sugar introduced into the fermentation medium is consumed by said micro-organisms, PA1 b) some of the fermentation medium obtained in this way is transferred into a second fermentation area including at least one fermenter, the volume being maintained constant in the first fermentation area by adding sugar, PA1 c) production of arabitol continues in said second fermentation area in such a manner as to consume the residual sugar of the fermentation medium, PA1 d) the fermentation medium thus obtained from said second fermentation area is separated continuously into a fraction concentrated in micro-organisms and another, soluble fraction enriched in arabitol, and PA1 e) the arabitol thus obtained is collected.
The first step of the process according to the invention consists of producing arabitol in a first fermentation area comprising at least one fermenter in such a manner that part of the sugar introduced into the fermentation medium is consumed by said micro-organisms.
The micro-organisms are advantageously selected from natural or modified osmosis-tolerant yeasts producing arabitol from sugar as a directly assimilable source of carbon.
The term "osmosis-tolerant yeasts" refers to yeasts which can withstand high osmotic pressures and are of the genus Yamadazyma, Debaromyces, Hansenula, Candida, Zygosaccharomyces or Saccharomyces. The above list is not limiting on the invention.
The term "natural osmosis-tolerant yeasts" refers to osmosis-tolerant yeasts which are isolated from their environment for their natural capacity for producing a given metabolite, here arabitol.
The term "modified osmosis-tolerant yeasts" refers to osmosis-tolerant yeasts whose natural capacity for producing a given metabolite, here arabitol, has been optimised by the use of random or controlled mutagenesis techniques or the techniques of molecular biology.
The term "sugar" refers in the context of the present invention to all carbonaceous sources which can be assimilated directly by the arabitol-producing micro-organisms.
The sugar is selected from the group consisting of glucose, galactose, sucrose, arabinose, glycerol, fructose, maltose, xylulose, ribulose, mannitol, myo-inositol, ethanol, starch and maltose, for example, alone or in combination.
The first fermentation area is adapted to provide a high production of arabitol by encouraging the consumption of some of the sugar introduced into the fermentation medium.
In the context of the invention, the expression "some of the sugar" means at least 50 wt. % of the sugar, preferably at least 80 wt. % and even more preferably at least 90 wt. %.
In a preferred first embodiment of the method according to the invention, production of arabitol is initiated in the first fermentation area in the batch mode, and all the fermentation substrates, including the sugar, and the micro-organisms are introduced simultaneously.
In a second preferred embodiment of the method according to the invention, production of arabitol is initiated in the first fermentation area in the fed batch mode and the arabitol-producing micro-organisms are fed with sugar progressively.
A fed batch mode is advantageously chosen to start fermentation in the method according to the invention.
This batch feeding mode progressively fills the first fermentation area with substrates to maintain a high rate of growth of the micro-organisms, good conversion of the sugar into arabitol and a sugar concentration in the fermentation medium tolerated by the arabitol-producing micro-organism concerned.
The expression "sugar concentration tolerated by the micro-organism" refers to a minimum sugar concentration which does not inhibit either the growth of said micro-organism or its production of arabitol.
Generally speaking, for all arabitol-producing micro-organisms, the process conditions are adjusted to avoid total consumption of the sugar.
The high production of arabitol in the first fermentation area generally leads to relatively high quantities of unconsumed residual sugar, depending on the micro-organisms used. As a general rule, the residual sugar content is at least 2.5 wt. %.
The second step of the method according to the invention then consists of transferring some of the fermentation medium obtained in this way into a second fermentation area comprising at least one fermenter, the volume being maintained constant in the first fermentation area by adding sugar.
Thus some of the fermentation medium is transferred from the first fermentation area to the second fermentation area when said fermentation medium has been enriched with arabitol but still contains unconsumed residual sugar in an amount which renders purification of the arabitol produced complex and costly.
This transfer is accompanied by continuous feeding of sugar into the first fermentation area, not only to maintain the volume constant, but also, and most importantly, to continue the production of arabitol with a productivity and a conversion yield at least equal to the values obtained during the arabitol production initiation phase described above.
The first fermentation area is advantageously continuously fed with the substrates, rather than with sugar, as described hereinafter.
The third step of the method according to the invention consists of continuing the production of arabitol in the second fermentation area in such a manner as to consume the residual sugar in the portion of the fermentation medium from the first fermentation area.
The second fermentation area is therefore used both to continue the production of arabitol and also, and most importantly, to complete the consumption of the sugar contaminating the portion of the fermentation medium transferred from the first fermentation area to the second fermentation area in such a manner as to leave only a low residual sugar content.
A low residual sugar content advantageously means a sugar concentration in the fermentation medium not greater than 2 wt. % and preferably not greater than 1 wt. %
In a preferred embodiment of the method according to the invention, this step is implemented in different size fermenters, the fermenters of the second fermentation area having a smaller volume than those of the first fermentation area.
In an even more preferred embodiment of the method according to the invention, there is only one fermenter in each fermentation area.
The fourth step of the method according to the invention consists of continuously separating the fermentation medium with a low residual sugar content obtained in this way into a fraction concentrated in micro-organisms and another, soluble fraction enriched in arabitol.
Continuous separation of the micro-organisms and the arabitol-enriched fraction is effected by any means known to the skilled person, for example by micro-filtration, using membranes with a pore diameter matched to the size of the micro-organism concerned, or by centrifuging at from 1000 to 10000 g, preferably by micro-filtration, as described hereinafter.
The arabitol-enriched clarified solution obtained at the end of the separation step constitutes the arabitol produced.
The Applicant company has also established that it can be advantageous to recycle the fraction enriched in micro-organisms to the entry of the first fermentation area.
The advantage of this micro-organism recycling step is to re-introduce a biomass whose fermentation capacities are complete and thereby to avoid the time-delay that would result from preparing a new culture of arabitol-producing micro-organisms.
The arabitol produced is recovered by concentration to a value of the order of 20% or more and this can be followed by a crystallisation step using any method known to the skilled person.